Conventional hearing protection earplugs of this type are designed such that they close off the ear canal as completely as possible, in order to achieve maximum sound attenuation in this manner.
There are numerous fields of application, however, where even though it is necessary and desirable to attain a sound reduction, it is nonetheless important to maintain a means of communication through speech, or hear ambient noises or warning signals.
Another problem in conventional hearing protection earplugs consists in that they generate disturbing sounds, which are in particular otoacoustic emissions, bone conductions, or a transmission of one's own speech into the ear.
Such otoacoustic emissions are sounds that are generated within the ear itself and that accordingly practically emanate from within the ear. The emission occurs spontaneously or in response to an external acoustic stimulus. These sounds are generated by the hair cells of the organ of Corti in the cochlea and are emitted retrograde against the primary direction of the flow of sound, which is directed into the ear. This means that the emission leaves the organ of Corti via the oval window and accordingly also via the inner ear, in order to then pass through the middle ear via the auditory ossicles and enter via the eardrum into the outer ear canal, where they are then detectable by very sensitive measuring microphones.
Bone conduction refers to the conduction of sound vibrations through the bone substance surrounding the organ of hearing while bypassing the middle ear. Because of the high acoustic resistance of the cranial bone, the conscious perception of the bone conduction is usually masked by signals that are transmitted as airborne sound. When hearing protection is used, bone conduction leads to a distortion of sounds.
This so-called occlusion effect, which, besides bone conduction, is based also on tissue conduction, results in the user hearing his own voice not only distorted but also amplified, in the same way as other body sounds, such as e.g. breathing and blood flow, which has the effect that a certain block can be observed in the user, or isolation.
In DE 20 2005 009 132 U, WO 2005/122981 A1 and WO 2004/064672 A2, a hearing protection earplug is described in each case that has a hollow at one of its axial end faces. Another hearing protection earplug that is described in DE 202 00 638 U1 is designed substantially hollow and provided at one axial end face with a diaphragm. From DE 93 02 783 U1, DE 73 14 145 U1, DE 690 13 927 T2 and U.S. Pat. No. 6,484,842 B1, hearing protection earplugs are known, some of them comprising multiple parts, that have an axially continuous longitudinal hollow in each case, the opening widths of which can vary in the longitudinal direction. Additionally, hearing protection earplugs are disclosed in DE 690 13 927 T2, U.S. Pat. Nos. 1,016,877 and 6,484,842 B1 that are provided with hollows at their lateral circumferential surface.